The present invention relates to lithographic printing.
Lithographic printing, also known as offset printing, is a printing process wherein an image is made upon a blanket cylinder and then transferred to a substrate being printed. The inked image is made upon the blanket cylinder by a printing plate having image and non-image areas thereon. The printing plate is dampened with a fountain solution provided from a dampening system to coat the non-image areas. Ink is provided to the printing plate from an inking system, and coats the image areas of the printing plate.
With known lithographic printing processes, changes in printing press performance and printing results are effected through variation of the ink formulations. Catalytic agents are introduced directly to the ink. Printing inks have been modified in this way for various specific requirements such as adhesion, abrasion resistance, ductility, hardness, opacity, transparency, color, shading, gloss and density.
For example, U.S. Pat. No. 3,682,688 to Hughes et al. discloses adding a compound to lithographic printing ink to accelerate its drying. U.S. Pat. Nos. 4,659,848 and 4,705,568 both to Kay et al., add compounds to lithographic printing ink to promote its adhesion to plastic substrates. Varying the ink formulation to improve the inks performance, however, requires a costly specialty ink manufactured for the particular result desired. Without such an ink, the known processes are restricted to the off-the-shelf, commercially available inks.
Accordingly, it is desirable to introduce the above-mentioned catalytic agents to the ink through the fountain solution of the dampening system in order to avoid the need for a specialty ink. Fountain solution additives are known for lithographic printing. However, these additives do not impart any of the above-described qualities to the ink. Rather, they merely improve upon the basic function of the fountain solution, i.e., keeping the plate clean.
Other problems also exist in the known lithographic printing processes. At the exit of the known lithographic printing devices, either a pile of printed sheets or a roll of printed web is accumulated. If the ink offset is not dry it may transfer from the surface of a freshly printed sheet or web portion to the back of the sheet or web portion on top of it. This transfer of ink is known as "ink offset."Ink offset reduces the printed image quality, and stains the adjacent sheet or web portion with ink.
Presently, it is known to reduce or substantially eliminate ink offset by placing an anti-offset spray powder or a coating (which also may give improved gloss, etc.) over the printed sheet or web before its accumulation in a pile or roll. The spray powder migrates to other areas of the offset lithographic printing device. Constant maintenance is required to avoid damage to these other areas by the powder. Also, additional equipment is required to apply the powder. Similarly, anti-offset coatings require additional equipment for their application and often require additional drying equipment.